Possibly tower bridge?

[irach] Phone booth sized? NO
[FGZ] Bridge? NO

[Rosie] A tunnel? NO, but a tunnel might be incorporated.

Possibly the one that must not be mentioned as it would cause a game end sequence?

[GL] A sewer? NO
[FGZ] Tube station? NO *audience starts to lose interest*

[irach] high- or rail-way? NO

The words on the card relate to something that is Mineral, often with a little vegetable
Positives established are:

• The vegetable portion is wood
• It is manufactured
• bigger than a 4-slice toaster in all 3 dimensions
• primarily functional
• made of concrete
• A person can "go inside"
• some other kind of "structure" (other than building or statue/sculpture)
• Usually outside

Negatives established are:
No precious minerals; not naturally occurring; no accidental vegetable material; not a geographical feature; not a building, not unique; not a garden object; not made of stone; can't be picked up in one hand; not a signal; doesn't start with P; not a steam locomotive; not seen in groups; not a bollard; not industrial; not a shed/barn; not a shelter; not phone-box-sized; not a bridge; not a tunnel per se (but may incorporate a tunnel); not a sewer; not a tube station; not a highway nor a railway.

The audience were appreciative of it being outdoors, primarily functional and made of concrete. They were given to debate amongst themselves as to whether a person could go inside, or whether it was a form of structure. I am, however, confident of saying YES to both of these.

Possibly a bench or picnic table?

[Raak] Subway? NO (MAY incorporate a tunnel, but not in the majority of cases)
[FGZstar] Outdoor furniture? NO (a person can go inside?)

[FGZstar] A dam? NO (it's not industrial)
[GL] Cooling tower? NO (ditto)
[INJ] Military? NO

[jim] Hmmm...*reconsults dictionary*

Hidden text

inside (n): on the inner side or part of; within

Would a person normally go inside it? YES is the potentially misleading, but correct, answer.

Not sure if it would count as industrial, so a grain silo?

[FGZstar] Agricultural? NO
[GL] Concrete overcoat? NO
[Raak] Made before 20th century? NO *Audience re-awakens*
[INJ] "inside" = "indoors"? NO *applause*
[Rosie] Cellar? NO
[FGZstar] Concrete boots? NO

[Rosie] Sports stadium? Using Wordnet's definition

Hidden text

a large structure for open-air sports or entertainments

YES (other dictionaries would lead to "NO, not strictly" being the answer) *Best applause yet!*
[Dujon] Winter Olympics? NO
[Raak] A wall? NO

As in skateboarding

[GL] A half-pipe? NO *rapturous and enthusiastic applause in anticipation of an early conclusion*

Blatantly taking off Gusset Login's guess, here.

[Software] A sewer? Hmmmm...that's still a NO, as nothing has changed since Saturday :-)

[jim] We have a winner! - a skatepark (or skate park) it is!

Hidden text

I got a skateboard for my birthday, which I'm calling a midlife crisis on a budget. I used to be quite handy on a skateboard in the late 70s, but kick-flips, ollies etc hadn't been invented then

So, one baton with its jeans hanging off passed on to jim

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[Phil] Just a quick query - why is that concrete and wood? The one I know best is all metal - I suppose the wood could be the boards.....

[INJ] Metal skatepark...hmmm I've not come across one. All the ones I know are concrete, with the odd wooden ramp thrown in for old-times' sake (plus some trees and fencing). I've now done some further searching and found that you are right. I was working mainly from a quote from a skateboarding magazine that concrete is the industry standard these days. My apologies if that threw people completely off the scent.